When a class is loaded and initialized in JVM - Java

Classloading and initialization in Java

Understanding of when a class is loaded and initialized in JVM is one of the fundamental concept of Java programming language. Thanks to Java language specification we have everything clearly documented and explained, but many Java programmer still doesn't know when a class is loaded or when a class is initialized in Java. Class loading and initialization seems confusing and complex to many beginners and its true until having some experience in belt its not always easy to get into subtle details of How JVM works in Java. In this Java tutorial we will see when class loading occurs in Java and when and how class and interface are initialized in Java. I will not go into detail of ClasLoader or How ClassLoader works in  Java, that is subject of another post I am planning just to keep this article focused and concise. There are several articles on Java fundamentals in Javarevisited like How HashMap works in Java and How Garbage collection works in Java. If you are interested you can also check those.

When Class is loaded in Java

Class loading is done by ClassLoaders in Java which can be implemented to eagerly load a class as soon as another class references it or lazy load the class until a need of class initialization occurs. If Class is loaded before its actually being used it can sit inside before being initialized. I believe this may vary from JVM to JVM. While its guaranteed by JLS that a class will be loaded when there is a need of static initialization.

When a Class is initialized in Java

After class loading, initialization of class takes place which means initializing all static members of class. A Class is initialized in Java when :

1) an Instance of class is created using either new() keyword or using reflection using class.forName(), which may throw ClassNotFoundException in Java.

2) an static method of Class is invoked.

3) an static field of Class is assigned.

4) an static field of class is used which is not a constant variable.

5) if Class is a top level class and an assert statement lexically nested within class is executed.

Reflection can also cause initialization of class. Some methods of java.lang.reflect package may cause class to be initialized. JLS Strictly says that a class should not be initialized by any reason other than above.

How Class is initialized in Java

Now we know what triggers initialization of a class in Java, which is precisely documented in Java language specification. Its also important to know in which order various fields (static and non static), block (static an non static), various classes (sub class and super class) and various interfaces (sub interface, implementation class and super interface) is initialized in Java. Infact many Core Java interview question and SCJP question based on this concept because it affect final value of any variable if its initialized on multiple places. Here are some of the rules of class initialization in Java:

1) Classes are initialized from top to bottom so field declared on top initialized before field declared in bottom

2) Super Class is initialized before Sub Class or derived class in Java

3) If Class initialization is triggered due to access of static field, only Class which has declared static field is initialized and it doesn't trigger initialization of super class or sub class even if static field is referenced by Type  of Sub Class, Sub Interface or by implementation class of interface.

4) interface initialization in Java doesn't cause super interfaces to be initialized.

5) static fields are initialized during static initialization of class while non static fields are initialized when instance of class is created. It means static fields are initialized before non static fields in Java.

6)non static fields are initialized by constructors in Java. sub class constructor implicitly call super class constructor before doing any initialization, which guarantees that non static or instance variables of super class is initialized before sub class.

Examples of  class initialization in Java:

Here is an example of when class is initialized in Java. In this example we will see which classes are initialized in Java.

/**
 * Java program to demonstrate class loading and initialization in Java.
 */
public class ClassInitializationTest {

    public static void main(String args[]) throws InterruptedException {
 
        NotUsed o = null; //this class is not used, should not be initialized
        Child t = new Child(); //initializing sub class, should trigger super class initialization
        System.out.println((Object)o == (Object)t);
    }
}

/**
 * Super class to demonstrate that Super class is loaded and initialized before Subclass.
 */
class Parent {
    static { System.out.println("static block of Super class is initialized"); }
    {System.out.println("non static blocks in super class is initialized");}
}

/**
 * Java class which is not used in this program, consequently not loaded by JVM
 */
class NotUsed {
    static { System.out.println("NotUsed Class is initialized "); }
}

/**
 * Sub class of Parent, demonstrate when exactly sub class loading and initialization occurs.
 */
class Child extends Parent {
    static { System.out.println("static block of Sub class is initialized in Java "); }
    {System.out.println("non static blocks in sub class is initialized");}
}

Output:
static block of Super class is initialized
static block of Sub class is initialized in Java
non static blocks in super class is initialized
non static blocks in sub class is initialized
false

Observation:

1) Super class is initialized before sub class in Java.

2) Static variables or blocks are initialized before non static blocks or fields.

3) Not used class is not initialized at all because its not been used, none of the cases mentioned on JLS or above which triggers initialization of class is not happened here.

Let's have a look on another example of class initialization in Java:

/**
 * Another Java program example to demonstrate class initialization and loading in Java.
 */

public class ClassInitializationTest {

    public static void main(String args[]) throws InterruptedException {
 
       //accessing static field of Parent through child, should only initialize Parent
       System.out.println(Child.familyName);
    }
}

class Parent {
    //compile time constant, accessing this will not trigger class initialization
    //protected static final String familyName = "Lawson";
 
    protected static String familyName = "Lawson";
 
    static { System.out.println("static block of Super class is initialized"); }
    {System.out.println("non static blocks in super class is initialized");}
}

Output:
static block of Super class is initialized
Lawson

Observation

1. Here class initialization occurs because static field is accessed which is not a compile time constant. had you declare "familyName" compile time constant using final keyword in Java (as shown in commented section) class initialization of super class would not have occurred.

2) Only super class is initialized even though static field is referenced using sub type.

There is another example of class initialization related to interface on JLS which explains clearly that initialization of sub interfaces does not trigger initialization of super interface. I highly recommend reading JLS 14.4 for understating  class loading and initialization in more detail.

That's all on When a class is initialized and loaded in Java. We have seen clear guidelines form JLS regarding class initialization. We have also seen the order on which super type and sub type are initialized and order of initialization for both static and non static fields and blocks in Java.

Standard Edition of Java Platform

Packages
java.applet	Provides the classes necessary to create an applet and the classes an applet uses to communicate with its applet context.
java.awt	Contains all of the classes for creating user interfaces and for painting graphics and images.
java.awt.color	Provides classes for color spaces.
java.awt.datatransfer	Provides interfaces and classes for transferring data between and within applications.
java.awt.dnd	Drag and Drop is a direct manipulation gesture found in many Graphical User Interface systems that provides a mechanism to transfer information between two entities logically associated with presentation elements in the GUI.
java.awt.event	Provides interfaces and classes for dealing with different types of events fired by AWT components.
java.awt.font	Provides classes and interface relating to fonts.
java.awt.geom	Provides the Java 2D classes for defining and performing operations on objects related to two-dimensional geometry.
java.awt.im	Provides classes and interfaces for the input method framework.
java.awt.im.spi	Provides interfaces that enable the development of input methods that can be used with any Java runtime environment.
java.awt.image	Provides classes for creating and modifying images.
java.awt.image.renderable	Provides classes and interfaces for producing rendering-independent images.
java.awt.print	Provides classes and interfaces for a general printing API.
java.beans	Contains classes related to developing beans -- components based on the JavaBeansTM architecture.
java.beans.beancontext	Provides classes and interfaces relating to bean context.
java.io	Provides for system input and output through data streams, serialization and the file system.
java.lang	Provides classes that are fundamental to the design of the Java programming language.
java.lang.annotation	Provides library support for the Java programming language annotation facility.
java.lang.instrument	Provides services that allow Java programming language agents to instrument programs running on the JVM.
java.lang.management	Provides the management interface for monitoring and management of the Java virtual machine as well as the operating system on which the Java virtual machine is running.
java.lang.ref	Provides reference-object classes, which support a limited degree of interaction with the garbage collector.
java.lang.reflect	Provides classes and interfaces for obtaining reflective information about classes and objects.
java.math	Provides classes for performing arbitrary-precision integer arithmetic (BigInteger) and arbitrary-precision decimal arithmetic (BigDecimal).
java.net	Provides the classes for implementing networking applications.
java.nio	Defines buffers, which are containers for data, and provides an overview of the other NIO packages.
java.nio.channels	Defines channels, which represent connections to entities that are capable of performing I/O operations, such as files and sockets; defines selectors, for multiplexed, non-blocking I/O operations.
java.nio.channels.spi	Service-provider classes for the java.nio.channels package.
java.nio.charset	Defines charsets, decoders, and encoders, for translating between bytes and Unicode characters.
java.nio.charset.spi	Service-provider classes for the java.nio.charset package.
java.rmi	Provides the RMI package.
java.rmi.activation	Provides support for RMI Object Activation.
java.rmi.dgc	Provides classes and interface for RMI distributed garbage-collection (DGC).
java.rmi.registry	Provides a class and two interfaces for the RMI registry.
java.rmi.server	Provides classes and interfaces for supporting the server side of RMI.
java.security	Provides the classes and interfaces for the security framework.
java.security.acl	The classes and interfaces in this package have been superseded by classes in the java.security package.
java.security.cert	Provides classes and interfaces for parsing and managing certificates, certificate revocation lists (CRLs), and certification paths.
java.security.interfaces	Provides interfaces for generating RSA (Rivest, Shamir and Adleman AsymmetricCipher algorithm) keys as defined in the RSA Laboratory Technical Note PKCS#1, and DSA (Digital Signature Algorithm) keys as defined in NIST's FIPS-186.
java.security.spec	Provides classes and interfaces for key specifications and algorithm parameter specifications.
java.sql	Provides the API for accessing and processing data stored in a data source (usually a relational database) using the JavaTM programming language.
java.text	Provides classes and interfaces for handling text, dates, numbers, and messages in a manner independent of natural languages.
java.text.spi	Service provider classes for the classes in the java.text package.
java.util	Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).
java.util.concurrent	Utility classes commonly useful in concurrent programming.
java.util.concurrent.atomic	A small toolkit of classes that support lock-free thread-safe programming on single variables.
java.util.concurrent.locks	Interfaces and classes providing a framework for locking and waiting for conditions that is distinct from built-in synchronization and monitors.
java.util.jar	Provides classes for reading and writing the JAR (Java ARchive) file format, which is based on the standard ZIP file format with an optional manifest file.
java.util.logging	Provides the classes and interfaces of the JavaTM 2 platform's core logging facilities.
java.util.prefs	This package allows applications to store and retrieve user and system preference and configuration data.
java.util.regex	Classes for matching character sequences against patterns specified by regular expressions.
java.util.spi	Service provider classes for the classes in the java.util package.
java.util.zip	Provides classes for reading and writing the standard ZIP and GZIP file formats.
javax.accessibility	Defines a contract between user-interface components and an assistive technology that provides access to those components.
javax.activation
javax.activity	Contains Activity service related exceptions thrown by the ORB machinery during unmarshalling.
javax.annotation
javax.annotation.processing	Facilities for declaring annotation processors and for allowing annotation processors to communicate with an annotation processing tool environment.
javax.crypto	Provides the classes and interfaces for cryptographic operations.
javax.crypto.interfaces	Provides interfaces for Diffie-Hellman keys as defined in RSA Laboratories' PKCS #3.
javax.crypto.spec	Provides classes and interfaces for key specifications and algorithm parameter specifications.
javax.imageio	The main package of the Java Image I/O API.
javax.imageio.event	A package of the Java Image I/O API dealing with synchronous notification of events during the reading and writing of images.
javax.imageio.metadata	A package of the Java Image I/O API dealing with reading and writing metadata.
javax.imageio.plugins.bmp	Package containing the public classes used by the built-in BMP plug-in.
javax.imageio.plugins.jpeg	Classes supporting the built-in JPEG plug-in.
javax.imageio.spi	A package of the Java Image I/O API containing the plug-in interfaces for readers, writers, transcoders, and streams, and a runtime registry.
javax.imageio.stream	A package of the Java Image I/O API dealing with low-level I/O from files and streams.
javax.jws
javax.jws.soap
javax.lang.model	Classes and hierarchies of packages used to model the Java programming language.
javax.lang.model.element	Interfaces used to model elements of the Java programming language.
javax.lang.model.type	Interfaces used to model Java programming language types.
javax.lang.model.util	Utilities to assist in the processing of program elements and types.
javax.management	Provides the core classes for the Java Management Extensions.
javax.management.loading	Provides the classes which implement advanced dynamic loading.
javax.management.modelmbean	Provides the definition of the ModelMBean classes.
javax.management.monitor	Provides the definition of the monitor classes.
javax.management.openmbean	Provides the open data types and Open MBean descriptor classes.
javax.management.relation	Provides the definition of the Relation Service.
javax.management.remote	Interfaces for remote access to JMX MBean servers.
javax.management.remote.rmi	The RMI connector is a connector for the JMX Remote API that uses RMI to transmit client requests to a remote MBean server.
javax.management.timer	Provides the definition of the Timer MBean.
javax.naming	Provides the classes and interfaces for accessing naming services.
javax.naming.directory	Extends the javax.naming package to provide functionality for accessing directory services.
javax.naming.event	Provides support for event notification when accessing naming and directory services.
javax.naming.ldap	Provides support for LDAPv3 extended operations and controls.
javax.naming.spi	Provides the means for dynamically plugging in support for accessing naming and directory services through thejavax.naming and related packages.
javax.net	Provides classes for networking applications.
javax.net.ssl	Provides classes for the secure socket package.
javax.print	Provides the principal classes and interfaces for the JavaTM Print Service API.
javax.print.attribute	Provides classes and interfaces that describe the types of JavaTM Print Service attributes and how they can be collected into attribute sets.
javax.print.attribute.standard	Package javax.print.attribute.standard contains classes for specific printing attributes.
javax.print.event	Package javax.print.event contains event classes and listener interfaces.
javax.rmi	Contains user APIs for RMI-IIOP.
javax.rmi.CORBA	Contains portability APIs for RMI-IIOP.
javax.rmi.ssl	Provides implementations of RMIClientSocketFactory and RMIServerSocketFactory over the Secure Sockets Layer (SSL) or Transport Layer Security (TLS) protocols.
javax.script	The scripting API consists of interfaces and classes that define Java TM Scripting Engines and provides a framework for their use in Java applications.
javax.security.auth	This package provides a framework for authentication and authorization.
javax.security.auth.callback	This package provides the classes necessary for services to interact with applications in order to retrieve information (authentication data including usernames or passwords, for example) or to display information (error and warning messages, for example).
javax.security.auth.kerberos	This package contains utility classes related to the Kerberos network authentication protocol.
javax.security.auth.login	This package provides a pluggable authentication framework.
javax.security.auth.spi	This package provides the interface to be used for implementing pluggable authentication modules.
javax.security.auth.x500	This package contains the classes that should be used to store X500 Principal and X500 Private Crendentials in aSubject.
javax.security.cert	Provides classes for public key certificates.
javax.security.sasl	Contains class and interfaces for supporting SASL.
javax.sound.midi	Provides interfaces and classes for I/O, sequencing, and synthesis of MIDI (Musical Instrument Digital Interface) data.
javax.sound.midi.spi	Supplies interfaces for service providers to implement when offering new MIDI devices, MIDI file readers and writers, or sound bank readers.
javax.sound.sampled	Provides interfaces and classes for capture, processing, and playback of sampled audio data.
javax.sound.sampled.spi	Supplies abstract classes for service providers to subclass when offering new audio devices, sound file readers and writers, or audio format converters.
javax.sql	Provides the API for server side data source access and processing from the JavaTM programming language.
javax.sql.rowset	Standard interfaces and base classes for JDBC RowSet implementations.
javax.sql.rowset.serial	Provides utility classes to allow serializable mappings between SQL types and data types in the Java programming language.
javax.sql.rowset.spi	The standard classes and interfaces that a third party vendor has to use in its implementation of a synchronization provider.
javax.swing	Provides a set of "lightweight" (all-Java language) components that, to the maximum degree possible, work the same on all platforms.
javax.swing.border	Provides classes and interface for drawing specialized borders around a Swing component.
javax.swing.colorchooser	Contains classes and interfaces used by the JColorChooser component.
javax.swing.event	Provides for events fired by Swing components.
javax.swing.filechooser	Contains classes and interfaces used by the JFileChooser component.
javax.swing.plaf	Provides one interface and many abstract classes that Swing uses to provide its pluggable look-and-feel capabilities.
javax.swing.plaf.basic	Provides user interface objects built according to the Basic look and feel.
javax.swing.plaf.metal	Provides user interface objects built according to the Java look and feel (once codenamed Metal), which is the default look and feel.
javax.swing.plaf.multi	Provides user interface objects that combine two or more look and feels.
javax.swing.plaf.synth	Synth is a skinnable look and feel in which all painting is delegated.
javax.swing.table	Provides classes and interfaces for dealing with javax.swing.JTable.
javax.swing.text	Provides classes and interfaces that deal with editable and noneditable text components.
javax.swing.text.html	Provides the class HTMLEditorKit and supporting classes for creating HTML text editors.
javax.swing.text.html.parser	Provides the default HTML parser, along with support classes.
javax.swing.text.rtf	Provides a class (RTFEditorKit) for creating Rich-Text-Format text editors.
javax.swing.tree	Provides classes and interfaces for dealing with javax.swing.JTree.
javax.swing.undo	Allows developers to provide support for undo/redo in applications such as text editors.
javax.tools	Provides interfaces for tools which can be invoked from a program, for example, compilers.
javax.transaction	Contains three exceptions thrown by the ORB machinery during unmarshalling.
javax.transaction.xa	Provides the API that defines the contract between the transaction manager and the resource manager, which allows the transaction manager to enlist and delist resource objects (supplied by the resource manager driver) in JTA transactions.
javax.xml	Defines core XML constants and functionality from the XML specifications.
javax.xml.bind	Provides a runtime binding framework for client applications including unmarshalling, marshalling, and validation capabilities.
javax.xml.bind.annotation	Defines annotations for customizing Java program elements to XML Schema mapping.
javax.xml.bind.annotation.adapters	XmlAdapter and its spec-defined sub-classes to allow arbitrary Java classes to be used with JAXB.
javax.xml.bind.attachment	This package is implemented by a MIME-based package processor that enables the interpretation and creation of optimized binary data within an MIME-based package format.
javax.xml.bind.helpers	JAXB Provider Use Only: Provides partial default implementations for some of the javax.xml.bind interfaces.
javax.xml.bind.util	Useful client utility classes.
javax.xml.crypto	Common classes for XML cryptography.
javax.xml.crypto.dom	DOM-specific classes for the javax.xml.crypto package.
javax.xml.crypto.dsig	Classes for generating and validating XML digital signatures.
javax.xml.crypto.dsig.dom	DOM-specific classes for the javax.xml.crypto.dsig package.
javax.xml.crypto.dsig.keyinfo	Classes for parsing and processing KeyInfo elements and structures.
javax.xml.crypto.dsig.spec	Parameter classes for XML digital signatures.
javax.xml.datatype	XML/Java Type Mappings.
javax.xml.namespace	XML Namespace processing.
javax.xml.parsers	Provides classes allowing the processing of XML documents.
javax.xml.soap	Provides the API for creating and building SOAP messages.
javax.xml.stream
javax.xml.stream.events
javax.xml.stream.util
javax.xml.transform	This package defines the generic APIs for processing transformation instructions, and performing a transformation from source to result.
javax.xml.transform.dom	This package implements DOM-specific transformation APIs.
javax.xml.transform.sax	This package implements SAX2-specific transformation APIs.
javax.xml.transform.stax	Provides for StAX-specific transformation APIs.
javax.xml.transform.stream	This package implements stream- and URI- specific transformation APIs.
javax.xml.validation	This package provides an API for validation of XML documents.
javax.xml.ws	This package contains the core JAX-WS APIs.
javax.xml.ws.handler	This package defines APIs for message handlers.
javax.xml.ws.handler.soap	This package defines APIs for SOAP message handlers.
javax.xml.ws.http	This package defines APIs specific to the HTTP binding.
javax.xml.ws.soap	This package defines APIs specific to the SOAP binding.
javax.xml.ws.spi	This package defines SPIs for JAX-WS.
javax.xml.ws.wsaddressing	This package defines APIs related to WS-Addressing.
javax.xml.xpath	This package provides an object-model neutral API for the evaluation of XPath expressions and access to the evaluation environment.
org.ietf.jgss	This package presents a framework that allows application developers to make use of security services like authentication, data integrity and data confidentiality from a variety of underlying security mechanisms like Kerberos, using a unified API.
org.omg.CORBA	Provides the mapping of the OMG CORBA APIs to the JavaTM programming language, including the class ORB, which is implemented so that a programmer can use it as a fully-functional Object Request Broker (ORB).
org.omg.CORBA_2_3	The CORBA_2_3 package defines additions to existing CORBA interfaces in the Java[tm] Standard Edition 6.   These changes occurred in recent revisions to the CORBA API defined by the OMG.  The new methods were added to  interfaces derived from the corresponding interfaces in the CORBA package.  This provides backward compatibility and avoids breaking the JCK tests.
org.omg.CORBA_2_3.portable	Provides methods for the input and output of value types, and contains other updates to theorg/omg/CORBA/portable package.
org.omg.CORBA.DynAnyPackage	Provides the exceptions used with the DynAny interface (InvalidValue, Invalid, InvalidSeq, andTypeMismatch).
org.omg.CORBA.ORBPackage	Provides the exception InvalidName, which is thrown by the method ORB.resolve_initial_references and the exception InconsistentTypeCode, which is thrown by the Dynamic Any creation methods in the ORB class.
org.omg.CORBA.portable	Provides a portability layer, that is, a set of ORB APIs that makes it possible for code generated by one vendor to run on another vendor's ORB.
org.omg.CORBA.TypeCodePackage	Provides the user-defined exceptions BadKind and Bounds, which are thrown by methods in in the class TypeCode.
org.omg.CosNaming	Provides a naming service for Java IDL.
org.omg.CosNaming.NamingContextExtPackage	This package contains the following classes, which are used in org.omg.CosNaming.NamingContextExt:
org.omg.CosNaming.NamingContextPackage	This package contains Exception classes for the org.omg.CosNaming package.
org.omg.Dynamic	This package contains the Dynamic module specified in the OMG Portable Interceptor specification,http://cgi.omg.org/cgi-bin/doc?ptc/2000-08-06, section 21.9.
org.omg.DynamicAny	Provides classes and interfaces that enable traversal of the data value associated with an any at runtime, and extraction of the primitive constituents of the data value.
org.omg.DynamicAny.DynAnyFactoryPackage	This package contains classes and exceptions from the DynAnyFactory interface of the DynamicAny module specified in the OMG The Common Object Request Broker: Architecture and Specification,http://cgi.omg.org/cgi-bin/doc?formal/99-10-07, section 9.2.2.
org.omg.DynamicAny.DynAnyPackage	This package contains classes and exceptions from the DynAny interface of the DynamicAny module specified in the OMG The Common Object Request Broker: Architecture and Specification, http://cgi.omg.org/cgi-bin/doc?formal/99-10-07, section 9.2.
org.omg.IOP	This package contains the IOP module specified in the OMG document The Common Object Request Broker: Architecture and Specification, http://cgi.omg.org/cgi-bin/doc?formal/99-10-07, section 13.6.
org.omg.IOP.CodecFactoryPackage	This package contains the exceptions specified in the IOP::CodeFactory interface (as part of the Portable Interceptors spec).
org.omg.IOP.CodecPackage	This package is generated from the IOP::Codec IDL interface definition.
org.omg.Messaging	This package contains the Messaging module specified in the OMG CORBA Messaging specification,http://cgi.omg.org/cgi-bin/doc?formal/99-10-07.
org.omg.PortableInterceptor	Provides a mechanism to register ORB hooks through which ORB services can intercept the normal flow of execution of the ORB.
org.omg.PortableInterceptor.ORBInitInfoPackage	This package contains the exceptions and typedefs from the ORBInitInfo local interface of thePortableInterceptor module specified in the OMG Portable Interceptor specification, http://cgi.omg.org/cgi-bin/doc?ptc/2000-08-06, section 21.7.2.
org.omg.PortableServer	Provides classes and interfaces for making the server side of your applications portable across multivendor ORBs.
org.omg.PortableServer.CurrentPackage	Provides method implementations with access to the identity of the object on which the method was invoked.
org.omg.PortableServer.POAManagerPackage	Encapsulates the processing state of the POAs it is associated with.
org.omg.PortableServer.POAPackage	Allows programmers to construct object implementations that are portable between different ORB products.
org.omg.PortableServer.portable	Provides classes and interfaces for making the server side of your applications portable across multivendor ORBs.
org.omg.PortableServer.ServantLocatorPackage	Provides classes and interfaces for locating the servant.
org.omg.SendingContext	Provides support for the marshalling of value types.
org.omg.stub.java.rmi	Contains RMI-IIOP Stubs for the Remote types that occur in the java.rmi package.
org.w3c.dom	Provides the interfaces for the Document Object Model (DOM) which is a component API of the Java API for XML Processing.
org.w3c.dom.bootstrap
org.w3c.dom.events
org.w3c.dom.ls
org.xml.sax	This package provides the core SAX APIs.
org.xml.sax.ext	This package contains interfaces to SAX2 facilities that conformant SAX drivers won't necessarily support.
org.xml.sax.helpers	This package contains "helper" classes, including support for bootstrapping SAX-based applications.

How to create read only List, Map and Set in Java – unmodifiable example

Read only List, Map and Set in Java

Read only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using get method or by using Iterator of List, This kind of List is good for certain requirement where parameters are final and can not be changed. In Java you can use Collections.unModifiableList() method  to create read only List , Collections.unmodifiableSet() for creating read-only Set like read only HashSet and similarly creating a read only Map in Java, as shown in below example. Any modification in read only List will result in java.lang.UnSupportedOperationException in Java.  This read-only List example is based on Java 5 generics but also applicable  to other Java version like JDK 1.4 or JDK 1.3, just remove Generics code i.e. angle bracket which is not supported prior to Java 5. One common mistake programmer makes is that assuming fixed size List and read only List as same. As shown in our 3 example of converting Array to Array List , we can use Arrays.asList() method to create and initialized List at same line. List implementation returned by this method is fixed size and it doesn’t allow adding or removal of element but it is not read only because you can update objects by callingset(index) method. How to make a collection read only is also a popular Java collection interview question, which makes this Java collection tutorial even more important.

Read only List, Set and Map Example - Java

Here is sample Java program which demonstrate method of creating read only List, Set and Map in Java. You can make any List, Set or Map implementation as read only by following code example. Just remember that Collections.unModifiableList() , Collections.unModifiableSet() and Collections.unModifiableMap() method in Java. On read only List add, remove and set operation is not permitted, on read only Set you can not add or remove elements and in read only Map you can not put new entries or update existing entries. You can use same methods to convert any List implementation like ArrayList, LinkedList or Vector to read only ArrayList , LinkedList and Vector in Java.

package example;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * Java program to create read only List, Set and Map in Java. You can first create

 * a List or Set and than make it unmodifiable or read-only by

 * using  Collections.unmodifiableList() or Collections.unmodifiableSet() method.
 *
 * @author Javin Paul
 */
public class ReadOnlyListSetMap {
 
    public static void main(String args[]) {            
       
        // creating List in Java
        List<String> contents = new ArrayList<String>();
     
        // initializing List in Java
        contents.add("Example");
        contents.add("Tutorial");
        contents.add("Program");
     
     
        // Currently This List is not read only, you can add or remove elements from List
        contents.add("Tips"); //should not be allowed if List is read only.
     
        System.err.println("normal List in Java : " + contents);
     
        //creating readonly List from contents
        contents = Collections.unmodifiableList(contents);
     
        //java.lang.UnsupportedOperationException -- no modification in read only list
     
       //not allowed as it is read-only List in Java 
       contents.add("Can I add object into read only List - No");

       
        contents.remove("Example"); //remove not allowed in read only list
     
        //java.lang.UnSupportedOperation - List update not allowed
        contents.set(0, "Can I override or set object in read-only Set - No");

     
        //Creating read only Set in Java
        //similar to read-only List you can also create a Set which is read only 
        //i.e. addition , removal and modification operation is not permitted on list
     
     
        //Creating a Set based on contents of List       
        Set<String> readOnlySet = new HashSet<String>(contents);
     
        System.out.println("original Set in Java : " + readOnlySet);

        //Set is not yet read-only you can still add elements into Set
        readOnlySet.add("Override");

        System.out.println("Set before making read only : " + readOnlySet);
     
        //making Set readonly in Java - no add remove or set operation permitted
        readOnlySet = Collections.unmodifiableSet(readOnlySet);
     
        //trying to add element in read only Set - java.lang.UnSupportedOperationException
        readOnlySet.add("You can not add element in read Only Set");
     
        //trying to remove element from read only set
        readOnlySet.remove("Example"); //you can not remove elements from read only Set
     
     
     
        // Creating read only Map in Java
        // Similar to List and Set you can also create read only or unmodifiable Map in Java
        // add , remove and override is not allowed on read only Map in Java
     
        Map<String, String> contries = new HashMap<String, String>();      
        contries.put("India", "New Delhi");
     
        //Map is not read only yet, you can still add entries into
        contries.put("UK", "London");
     
        System.out.println("Map in Java before making read only: " + contries);
     
        //Making Map read only in Java
        Map readOnlyMap = Collections.unmodifiableMap(contries);
       
        //you can not put a new entry in read only Map in Java
        readOnlyMap.put("USA", "Washington"); //java.lang.UnSupportedOperation 
       
        //you can not remove keys from read only Map in Java
        readOnlyMap.remove("UK"); //java.lang.UnSupportedOperation
     
    }
 
}

That’s all on how to create read only List, Set and Map in Java. Its very easy to make any List implementation read only by wrapping it with Collections.unModifiableList(). Use same technique to convert any other Collection into read only in Java.